An optical scanner has been known in relation to an image forming apparatus such as a digital copying machine, an optical printer, and an optical printing machine or the like. In the optical scanner as described above, there has been proposed a multi-beam optical scanning system for optically and concurrently scanning an image with a plurality of scanning lines for the purpose of speeding up an operation for writing images by way of optical scanning.
In the multi-beam optical scanning system, there is sometimes a case where scanning lines for optically and concurrently scanning are not adjacent to each other. There has been proposed, for instance, in Japanese Patent Publication No. HEI 6-418846, a so-called “interlace scanning” type of multi-beam optical scanner in which interlace scanning is executed by means of three scanning lines without one line therebetween.
In the interlace scanning as described above, selection of a signal for modulating each beam is irregular, so that optical scanning is easily complicated, and in addition, scanning lines for optically concurrently scanning are also largely spaced therebetween, so that “a rate of pitch deviation” of optical scanning due to a bend in the scanning lines becomes large, which makes it easy for image quality of a recorded image to degrade.
In the multi-beam optical scanner, there are strict restriction over a magnification in an image-formation system provided in a space between the light source and the surface for scanning due to a relation between pitches of scanning lines, and because of the restrictions over a magnification as described above, a position of an optical system to be provided in a side of the light source from the optical deflector must be closer to the optical deflector, and for this reason, a layout of optical arrangement becomes difficult.
Further, if a “hybrid” combination of two or more LD light emitting sections or LED light emitting sections is employed as a light source having a plurality of light emitting sections used in the multi-beam optical scanner, there occurs a problem of “wavelength deviation” that wavelengths of combined light emitting sections are not identical to each other, and when this wavelength deviation is present therein, constant velocity characteristics of optical scanning or the like may vary for each light emitting section.
What is known as a multi-beam optical scanning system is disclosed in Japanese Patent Publication No. HEI 7-111509, but in this optical scanning system a focal length of a lengthy lens for correcting surface offset is as short as 15 mm, and for this reason the lengthy lens is provided at a position close to the scanned surface, which makes larger a length in a direction corresponding to the main scanning as well as cost of the lengthy lens higher, and also toner splashed from a developing device generally provided adjacent to the scanned surface easily makes the lengthy lens contaminated.